Extended arm bifilar and method to eliminate second order vibration excitation

ABSTRACT

A helicopter has a plurality of bifilar absorbers mounted on arms radially extended from the hub of the rotor. A bifilar mass of the required weight mounted at the end of an arm of required length to absorb input excitations at a pendulum amplitude of approximately 30° at design forward speed is mounted on an arm twice the length without a reduction in weight of the bifilar mass to limit pendulum amplitude to 15° while maintaining the original force output. Bifilar motion in the linear range with associated elimination of overturning produces improved bifilar operation at all flight speeds and eliminates second order vibration due to dissimilar motions resulting from overtuning.

The invention herein described was made in the course of or under acontract or subcontract thereunder with the Department of the Army.

CROSS REFERENCE TO RELATED APPLICATIONS

In a copending application of I. Kenigsberg and E. Carter, Ser. No.631,577, filed Nov. 13, 1975, and assigned to the assignee of thisapplication, a bifilar absorber is shown which is related to thisapplication to the extent that the improvements disclosed in these twoapplications can be used together in the same basic bifilar absorberinstallation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the construction and use of bifilar absorbersas applied to helicopter rotors.

2. Description of the Prior Art

Prior to this invention bifilar absorbers as used on helicopter rotorshave operated at approximately ± 30° amplitude at design forward speed.To prevent undertuning at any speed up to design forward speed thebifilar absorbers have been overtuned by increasing the tuning pindiameters. This results in dissimilar pendulum motions which producerotating system vibratory excitations at twice the tuned frequency ofthe bifilar at all flight conditions except the design forward speed atwhich all of the masses of the bifilars reach an amplitude at whichovertuning no longer exists. Due to dissimilar motions of the severalabsorbers on the rotor, these excitations will excite the fixed system,including the fuselage, at a frequency of 2w_(n) -1, where w_(n) = thetuned frequency of the bifilar absorber at the design forward speed.

U.S. Pat. No. 3,540,809, issued to W. Paul et al, illustrates a priorart installation of bifilar absorbers on helicopter rotors. The presentinvention is directed to improvements in such installations.

SUMMARY OF THE INVENTION

It is a principal object of this invention to provide an improvedbifilar absorber installation and an improved method of operatingbifilar absorbers as installed on helicopter rotors in which theamplitude of the bifilar masses is limited to angles at or below ± 15°up to design forward speed, thereby minimizing overtuning of thebifilars, to improve performance at all speeds and elimination of mostdissimilar mass motions and the resulting second order vibrations.

Another object of this invention is to provide a helicopter rotor withbifilar absorbers in which bifilar motions are maintained in the linearrange with no reductions in the bifilar/airframe mass ratio which iscritical to bifilar effectiveness.

Still another object of this invention is to radially extend the bifilarabsorbers with no reduction in mass to minimize overtuning by limitingamplitude of the bifilars to angles below ± 15°.

The Paul U.S. Pat. No. 3,540,809 mentions, Col. 8, lines 62 to 68, thatthe bifilar radius can be doubled while halving the mass to produce thesame force output with no reduction in amplitude of motion. However,without a reduction in motion the bifilar would still require overtuningand thus produce the undesirable second order vibration and also reducebifilar effectiveness at all speeds except the design forward speed.

In furtherance of the above objects the length of the bifilar supportarm measured from the axis of rotation of the rotor to the center ofgravity of the bifilar mass and the mass of the bifilar required toabsorb excitations at pendulum amplitudes of approximately ± 30° isdefined. This mass without reduction in weight is then mounted on asupport arm of twice the length of the previous arm, thus halving thependular motion while maintaining the original force output. Since atthis reduced amplitude the bifilar motions are within the linear rangethe need for overtuning, with its inevitable second order vibrationexcitation to the fuselage, is eliminated.

A yet further object of this invention is generally to improve bifilarabsorber installations for helicopter rotors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial plan view of a helicopter rotor showing a prior artinstallation of bifilar vibration absorbers;

FIG. 2 is a sectional elevation showing an absorber supporting arm ofFIG. 1;

FIG. 2A is a similar view, drawn to the same scale, showing an absorbersupporting arm of the present invention;

FIG. 3 is a curve showing single pendulum amplitude plotted againstforward speed for the installation of FIG. 1;

FIG. 4 is a curve showing bifilar overtuning plotted against forwardspeed for the installation of FIG. 1;

FIG. 5 is a curve showing pendulum amplitude for the installation ofFIG. 1 plotted against airspeed in a helicopter rotor having fourbifilar absorbers,

FIG. 6 is a curve showing single pendulum amplitude plotted againstairspeed for the installation of FIG. 2A;

FIG. 7 is a curve comparing residual excitation plotted against forwardspeed for the installations of FIGS. 2 and 2A;

FIG. 8 is a curve similar to FIG. 5 for the improved bifilarinstallation of this invention; and

FIG. 9 is a curve showing second harmonic excitation plotted againstpendulum amplitude.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a prior art helicopter rotor including a rotor head 10rotatable about an upright axis 12 and equipped with a support member 14for a plurality of bifilar vibration absorbers 16, two of which areshown in this figure. Support member 14 includes a central ring 18equipped with depending lugs 20 by which it is fixed to rotor head 10 bybolts 22 for rotation therewith in a plane normal to axis 12. Ring 18has radially extending arms 24 which terminate in attaching portions 26for bifilar absorbers. Each absorber consists essentially of a pendulouselement including a mass 28 having similar upper and lower arms of whichonly the upper arm 30 is shown in FIG. 1 between which attaching portion26 of arm 24 is received. The arms of mass 28 and attaching portion 26of arm 24 have large, generally circular apertures 30a through whichtuning pins 32 of the absorbers extend. Apertures 30a may have bushings34, as indicated in FIG. 2, over which pins 32 move in a rolling fashionas the pendulous element swings.

FIG. 2A shows our new and improved bifilar support member which includesa central ring 38 equipped with two annular series of holes 40,42 bywhich it is attached to rotor hub 10 by bolts (not shown) and a longerradial arm 44 which terminates in a bifilar absorber attaching portion26a similar to portion 26 in FIG. 2.

Prior to this invention bifilar absorbers as used on helicopter rotorshave operated at approximately ± 30° amplitude at design forward speed.This is shown in the curve of FIG. 3. Due to the non-linear nature ofthe pendulum absorber, natural frequency will decrease with increasingamplitude. In order to prevent undertuning at any speed up to designforward speed, the bifilars have been initially overtuned. This isaccomplished by increasing the diameter of the tuning pins 32 andresults in the curve plotted in FIG. 4. This overtuning results indifferences an amplitude and phase between the motions of all bifilarmasses on the rotor at all flight conditions except the one in whichsufficient input force exists to produce an amplitude high enough toforce all of the bifilar masses to the amplitude at which all overtuningis eliminated. This situation is illustrated in FIG. 5 where pendulumamplitude is plotted against airspeed.

An additional disadvantage of the prior installation of FIGS. 1 and 2occurs due to the fact that the bifilar pendulums produce a secondharmonic force which, if all pendulum motions are not equal, willtransfer into the fixed system, including the fuselage, of thehelicopter at one harmonic of rotor speed less, than its frequency inthe rotating system i.e.

w_(n) pendulum = 3/rev.

2nd harmonic = 6/rev.

Fixed system excitation at 5/rev, where

w_(n) = the tuned frequency of the bifilar absorber, By this inventionthese difficulties caused by overtuning and the resulting secondharmonic forces are eliminated by providing an installation in which theamplitude of the bifilar masses is limited to angles at or below 15° ,thereby minimizing overtuning of the bifilars to improve performance atall speeds, as shown in FIG. 7, and elimination of most dissimilar massmotions and the resulting second order vibrations.

FIG. 9 shows very clearly the advantages of operating the bifilarabsorbers at or below a pendulum amplitude of 15° where the secondharmonic excitation is relatively low rather than, as prior to thisinvention, at 30° amplitude.

The bifilar absorber is first sized to determine the mass required toabsorb the input excitation force at ± 30° of motion. The bifilarsupport arm is then extended to reduce the motion to ± 15° with noreduction in mass. This extension of the bifilar support arm is roughlyequal to a doubling of the length of the arm in the FIG. 1 installationand eliminates overtuning requirements, maintains equal motions of theseveral bifilars of the installation and reduces any second harmonicforce by a significant amount. FIG. 8 shows the results of thisinvention, especially when compared with FIG. 5 which is representativeof the prior art bifilar installations.

While we have shown and described one embodiment of our invention, we donot wish to be limited to the exact details of construction shown anddescribed for obvious modifications will occur to a person skilled inthis art.

We claim:
 1. In a helicopter having a hub, a support member rotatablewith said hub having a plurality of radially extended arms, a bifilarabsorber mounted at the extremity of each arm having a pendulous mass,the length of said arms measured from the axis of rotation of said rotorto the c.g. of said masses and the weight of said masses being such asto absorb input excitations at design forward speed at pendulumamplitude of approximately ± 30°, that improvement comprising thecombination of said pendulous masses having the same weight with newradially extended supporting arms of substantially double the length ofsaid first mentioned arms.
 2. In a bifilar vibration absorber for ahelicopter rotor including a support arm of length n for a pendulousmass of sufficient weight of absorb input excitations at design forwardspeed at a pendulum amplitude of approximately ± 30°, that improvementcomprising the combination with said pendulous mass of means foreliminating second order excitations in the helicopter fuselage bycausing said absorber to operate in a more linear range at amplitude nogreater than ± 15° , said means comprising a new and longer arm oflength 2n replacing said arm n for supporting said pendulous mass withno reduction in weight of said mass.
 3. A bifilar absorber installationfor a helicopter rotor including a support member carried by said rotorhaving a plurality of radially extended arms of length n, each adaptedto support a bifilar absorber at its extended end, each absorberincluding a pendulous mass of sufficient weight to absorb inputexcitations at design forward speed at pendulum amplitude of ± 30° ,that improvement which consists in the combination with said pendulousmasses of means for eliminating dissimilar motions of the severalabsorbers on the rotor due to overtuning below design forward speedwhich excite the fuselage of the helicopter at a frequency 2w_(n) -1,where w_(n) = the tuned frequency of the absorbers at design forwardspeed and causing said absorbers to operate at pendulum amplitude of 3515°, said means consisting of new arms for supporting said pendulousmasses of substantially length 2n replacing said arms of length n.
 4. Ina vibration absorber for a helicopter rotor including a pendulous massof sufficient weight to absorb primary in-plane input excitationstransmitted from the rotor to said fuselage at design forward speed, andmeans for eliminating second order excitations in the helicopterfuselage, said means comprising a support arm for said pendulous mass ofsufficient length to cause said absorber to operate in a linear range atamplitude no greater than ± 15°.
 5. In a vibration absorber for ahelicopter rotor having a plurality of absorbers, each including apendulum mass of sufficient weight of absorb input excitations at designforward speed including means for eliminating dissimilar motions of theseveral absorbers on the rotors due to overtuning below design forwardspeed which excites the fuselage of the helicopter at a frequency 2w_(n)-1, where w_(n) = the tuned frequency of the absorbers at design forwardspeed, said means consisting of arms for supporting said means which areof sufficient length to limit the pendulum amplitude to less than ± 15°.6. In a helicopter rotor, a hub, a support member rotatable with saidhub having a plurality of radially extended arms, a bifilar absorbermounted at the extremity of each arm having a pendulous mass, the weightof said mass being sufficient to absorb the primary in-plane vibrationsof the rotor as determined by the dynamics of the rotor hub and saidarms having a length measured from the axis of rotation of said rotor tothe c.g. of said masses which limits the pendulum amplitude of saidabsorbers at design forward speed to ± 15°.